US Special Forces and FBI agents are in Syria searching for the remains of three Americans executed by ISIS and one woman who died while in captivity of unknown causes, according to a report from ABC News.

The people whose remains are being sought are journalists James Foley and Steven Sotloff and aid workers Abdul-Rahman (Peter) Kassig and Kayla Mueller.

Foley, Sotloff and Kassig were all beheaded by ISIS in 2014.

Mueller was killed in 2015, although ISIS claimed she was killed by a coalition bombing by a Jordanian warplane. This has not been confirmed by Jordanian or US officials.

Two captured ISIS members are reportedly being questioned for information on the whereabouts of the buried remains of the killed hostages.

The captured men are Alexanda Kotey and El Shafee Elsheikh, part of a four-person British contingent of the terrorist group nick-named “The Beatles.”

The leader of the “The Beatles,” Mohammed Emwazi, dubbed “Jihadi John,” was killed in 2015 by a drone strike.